This invention relates to apparatus for synchronizing an inverter-synchronous machine drive system and for maintaining such inverter-synchronous machine synchronism at light or zero synchronous machine load conditions.
In many industrial applications where operator control of synchronous machine speed is desired, synchronous machine drive systems are employed. Typically, such synchronous machine drive system comprise a synchronous machine and an inverter which is configured of a plurality of pairs of solid state switching devices, typically thyristors, each pair of thyristors being coupled in series aiding fashion, and each of the pairs coupled across a unidirectional current source. The junction between thyristors of each thyristor pair is coupled to a respective synchronous machine phase. Rendering the thyristors conductive in a predetermined sequence causes the synchronous machine to be supplied with alternating current and hence to commence rotation. By adjusting the frequency at which inverter thyristors conduct, the frequency of alternating current potential supplied to the synchronous machine and hence, synchronous machine speed, can be varied accordingly.
To achieve peak inverter-synchronous machine drive system performance, it is desirable to synchronize the inverter to the synchronous machine, that is, to adjust the frequency of inverter thyristor conduction to maintain commutation of inverter thyristors by synchronous machine back EMF. Such apparatus for achieving inverter-synchronous machine drive synchronism is described and claimed in the copending application Ser. No. 945,625, entitled "Method and Apparatus for Control of Inverter Synchronous Machine Drive Systems", filed by A. B. Plunkett and F. G. Turnbull on Sept. 25, 1978 and assigned to the assignee of the present invention. As taught in that application, inverter-synchronous machine drive system synchronization is achieved by adjusting the frequency of inverter thyristor conduction in accordance with a phase angle feedback signal which is proportional to the difference in magnitude between an operator-commanded machine phase angle relationship and the actual machine phase angle relationship between stator current and air gap flux.
While the method and apparatus described and claimed in the aforementioned application Ser. No. 945,625 achieves inverter-synchronous machine drive system synchronism over a wide range of machine load conditions, inverter-synchronous machine drive system synchronism may not always be maintained at light or zero machine load conditions. This is because machine stator current drawn during intervals of light or zero machine loads may be of insufficient magnitude to permit computation of the actual machine phase angle relationship magnitude. The apparatus of the present invention alleviates this difficulty thereby allowing inverter-synchronous machine drive system synchronism to be maintained even at light or zero machine load conditions.